side effect

UK: /ˈsaɪdɪfɛkt/

WordReference Random House Learner's Dictionary of American English © 2025

ˈside efˌfect, n. [countable]

1. an often harmful effect, as of a drug, secondary to the intended effect:
   The drug will stop the growth of the cancerous cells but the side effects are loss of hair and difficulty in eating.

WordReference Random House Unabridged Dictionary of American English © 2025

side′ effect′, 

1. any effect of a drug, chemical, or other medicine that is in addition to its intended effect, esp. an effect that is harmful or unpleasant.
2. any accompanying or consequential and usually detrimental effect:
   the side effects of air pollution.
Also, side′-ef•fect′. 
• 1880–85

Collins Concise English Dictionary © HarperCollins Publishers::

side effect noun

1. any unwanted nontherapeutic effect caused by a drug
2. any secondary effect, esp an undesirable one